The original way

Rename the file to "survival" (no quotes). The file should not have a file extension.

Navigate to the directory where Devil Daggers is installed and open the folder named "dd".
By default, this is C:\Program Files (x86)\Steam\steamapps\common\devildaggers\dd on Windows.

Replace the file called "survival" with the file you just downloaded.

NOTE: You can replace the "survival" file while Devil Daggers is running. Just make sure to go back to the main menu before playing again, otherwise it won't change. The game reloads the file every time you grab the dagger in the lobby.